An apparatus, system and method for flow rate harmonization in electric submersible pump (ESP) gas separators. A method for flow rate harmonization in ESP gas separators includes modifying flow of multi-phase well fluid through vent passages of a crossover when a flow rate of a centrifugal pump differs from a flow rate of a gas separator including the crossover, the gas separator serving as the fluid intake into the centrifugal pump. Flow of fluid through vent passages is modified by one of attaching flow sizing inserts into vent passages or production passages of the crossover, or by attaching a funnel to a crossover inlet. A gas separator system includes a series of interchangeable funnels attachable to a fluid entrance of a crossover of the gas separator, wherein interchanging the particular funnel attached to the crossover modifies flow rate output of the gas separator.